1. Technical Field
The present invention relates to a power unit having a main power source and an auxiliary power source, particularly using an electric double layer capacitor as an auxiliary power source and an image forming system.
2. Description of the Related Art
When supplying all power to a load having a peak from a main power source such as a battery or a DCPS (DC power supply system), it is necessary to fit the power supply capacity of the main power source to the peak. However, for that purpose, it causes enlargement of the apparatus such as enlarging the output transformer of the DCPS and an increase in cost.
On the other hand, to solve the aforementioned problem, a method for using a combination of an auxiliary power source using an electric double layer capacitor (hereinafter, simply referred to as a capacitor) with the main power source is proposed.
For example, in Japanese Unexamined Patent Application Publication No. 2003-250228, the system which has a power source controller for controlling a current supplied from the main power source between the power source and an energy storage unit having a capacitor, and when the output voltage of the power source lowers, the supply current from the power source is reduced, and insufficient power is covered by the capacitor is proposed.
Further, in Japanese Unexamined Patent Application Publication No. 7-75251, the system which uses a variable impedance such as a thermistor having a positive resistance temperature coefficient between the main power source and the capacitor, and when a large current flows through the load, using an increasing property of the variable impedance value, the current supply from the power source is suppressed, and the current is supplied exclusively from the capacitor is proposed.
However, in such a constitution that the internal impedance of the capacitor used as an auxiliary power source is higher than the internal impedance of the main power source, the capacitor cannot function sufficiently as an auxiliary power source.
This problem will be explained below.
In the capacitor, the internal impedance per cell is generally low, though the dielectric strength is low such as 2.5 V, so that when using the capacitor as an auxiliary power source as indicated in the above example, the dielectric strength must be increased, thus it is necessary to connect a plurality of capacitors in series. By doing this, for example, if 10 capacitors having an internal impedance of 100 mΩ of a cell having a dielectric strength of 2.5 V are connected in series, the dielectric strength is increased to 25 V/internal impedance of 1Ω.
On the other hand, in a nickel-hydrogen cell as a main power source, the internal impedance per cell (1.2 V) is 5 mΩ or so and when 21 cells are connected in series to obtain 25 V, the internal impedance becomes about 100 mΩ. On the other hand, when a DCPS is used as a main power source, the impedance thereof becomes about several tens mΩ to several hundreds mΩ.
Therefore, the internal impedance of the auxiliary power source may be higher than the internal impedance of the main power source.
When the internal impedance of the auxiliary power source becomes higher than the internal impedance of the main power source like this, at time of the peak load current, the current to be supplied originally from the auxiliary power source, since the internal impedance is high, is supplied almost from the main power source and a problem arises that the auxiliary power source does not fulfill its original function as an auxiliary power source. And, as a result, the charging energy of the capacitor cannot be used.
Furthermore, when supplying the current from the capacitor, a voltage drop due to the internal impedance occurs, so that another problem arises that the lowest operation voltage on the load side cannot be kept.
The present invention is proposed to solve the aforementioned problem and is intended to provide a power unit combined with a main power source and an auxiliary power source composed of an electric double layer capacitor for fulfilling the function of the auxiliary power source, using the charging energy stored in the capacitor, and furthermore maintaining the lowest operation voltage on the load side and an image forming system.